In a small bowl, whisk together milk, salt and egg yolk. Add milk mixture to butter, 1 tablespoon at a time, beating well after each addition. Add flour and mix just until combined. Scoop mixture out onto a piece of plastic wrap and press into a 4" circle. Cover well and chill for 1 hour.

Preheat oven to 350 degrees.

Remove dough from the refrigerator and unwrap. On a lightly floured surface, roll dough to a 10" circle. Gently transfer and fit dough into a 9" pie plate - place into the freezer and chill for 15 minutes. Place pie plate into the oven and bake until lightly browned, about 20 to 25 minutes. Remove and allow to cool.

To prepare filling

In a large skillet, heat oil over medium-high. Add onion and cook until softened, about 4 to 5 minutes. Stir in garlic and cook until fragrant, about 1 minute. Add spinach and cook until wilted, about 2 minutes.

In a medium bowl, whisk together milk, eggs, cheese, salt and nutmeg. Stir in spinach mixture. Pour filling into crust - place into the oven and bake until the center has set, about 30 to 35 minutes. Remove and allow to cool slightly before serving.